MONTERREY, Mexico, May 21 (Reuters) - Mexican conglomerate Alfa is considering how to finance its purchase of Canadian oil firm Pacific Rubiales, deciding between a bridge loan, and using the proceeds from an initial public offering of its auto unit Nemak, Alfa's finance chief said on Thursday.

Earlier, Pacific Rubiales Energy Corp, the largest private oil producer in Colombia, said it had accepted a C$6.50-per-share takeover bid from Alfa and investment firm Harbour Energy Ltd but said it might consider other offers.

Ramon Leal, Alfa's chief finance officer, told Reuters that the Mexican conglomerate was in the process of negotiating a three-year bridge loan, worth up to $530 million, with a financial institution, which could be used to finance the deal.

But, he added, "it's likely that even if we have the credit sorted out, the probability of using it is slim, because in the meantime, we're working on a Nemak share offering that ... could be ready for the market in June." (Reporting by Gabriela Lopez)